The layered disulfides (e.g. TiS.sub.2,MoS.sub.2, NbS.sub.2, etc.) have been studied extensively as a result of their propensity to intercalate both organic and inorganic substances into an interlayer van der Waals gap between adjacent sulfide layers. Such disulfides are described by M. Whittingham, in Prog. in Solid State Chem., 1978, page 41.
Metal-rich sulfides and selenides, such as Nb.sub.21 S.sub.8, Nb.sub.14 S.sub.5, Ta.sub.2 S,Ta.sub.6 S, Nb.sub.2 Se, and Ta.sub.2 Se, are known to exhibit different crystal structures. In particular, the coordinations for the metal and sulfur in the Nb-rich sulfides and Ta-rich sulfides are strikingly different. For example, in the Nb-rich sulfides, Nb.sub.21 S and Nb.sub.14 S.sub.5, the metal (Nb) coordinations are capped pentagonal prisms and capped distorted cubes while the coordinations of sulfur are capped trigonal prisms. On the other hand, in the Ta-rich sulfides, Ta.sub.2 S and Ta.sub.6 S, the structures can be viewed as Ta clusters consisting of linear chains of facing-sharing pentagonal antiprisms which interact via direct metal-to-metal interactions and via sulfur bridges.
The technical article by Dr. B. Harbrecht in Angew. Chem. Int. Ed. Engl. 1989, p. 1660, describes crystalline Ta.sub.2 Se that exhibits a layered structure comprising a sequence of Se-Ta-Ta-Ta-Ta-Se homoatomic layers in a body centered cubic (bcc) arrangement when the difference in the kinds of atoms involved is neglected. As is apparent, the homoatomic Ta layers (four in number) are interposed between layers of Se.